Vault 15 is one of the many underground survival facilities built by Vault-Tec, like Vault 13.
Long ago, the situation in Vault 15 led to a schism, and all the inhabitants split into four groups. The Jackals, the Vipers and the Khans turned into raiders that only wanted to kill everyone in the wasteland, but the fourth group founded Shady Sands that would later become the great and powerful capitol of NCR.
The four groups left this vault behind, and it is now populated with hostile Slags, leftover raider types from the old days.
In total, there are four computers inside that yield Electronic Parts if you use Science on them. There are two sub-levels with Lockers and two wall safes, all of which contain loot. Some of the loot is interesting and has combat value, well-worth the risks.
Vault 15 is located at Grid 39:27 (due east of Shady Sands).